Loudspeakers are integral/critical parts of all audio systems. However, loudspeakers are known as the weakest link in any sound reproduction scheme. Ideally, it should create a sound field proportional to the electric signal of the amplifier. Due to the physics of sound radiation, this paradigm has not been achieved, particularly in the low frequency region (<300 Hz). The low efficiency of the loudspeaker generates more heat than sound power output while adding undesired distortion to the output signal. The frequency response of a conventional loudspeaker usually rolls off faster at low frequencies (<300 Hz). Consequently, most loudspeaker systems employ more than one driver; such as subwoofers (very low frequencies); woofers (low frequencies); mid-range speakers; tweeters; and sometimes super-tweeters, to adequately reproduce a wide range of frequencies with even coverage. Also, the production of a good high-fidelity loudspeaker has required that the speakers be enclosed in a ported box, which acts like a Helmholtz resonator.
Miniaturization and integration of acoustic devices have been an important consideration in recent times. Consumer electronic devices, such as cell phones, laptops, tablets, and the like with more features and capabilities are ubiquitous and are positioning to become audio entertainment centers. However, they also exhibit severe audio deficiencies and pose many additional challenges to maintain the acoustic performance as enclosed acoustic volume size, power and membrane size are reduced significantly. Due to the smaller size of the speaker used in such devices, the low frequency response is severely affected. For example, as the size of the cell (or mobile) phone decreases, the volume of air behind the diaphragm is reduced. This small amount of volume behind the speaker limits the range of motion of the diaphragm. The speaker does not produce enough force to compress the air beyond a certain point, hence causing the air to push back. This reduces the displacement of the speaker diaphragm, which in turn lowers the output. Thus, low frequencies are affected the most by this phenomenon as the diaphragm moves with the largest amount of displacement at these frequencies. Consequently, the frequency response usually rolls off faster at low frequencies, herein referred to as “bass frequencies” which are those which are audible or able to be sensed by a human and are below 300 Hertz.
Thus, what is needed is a way to accurately reproduce sound when wavelengths far exceed that of the available length to propagate a wave.